I do not know what your problem could be but these cars, even my little 190e is stable on the highway at high speeds.
About a month ago, I had the same problem and to top it off, I had the alignment done a few days before. Anyway, I took it back to the shop and sure enough the alignment was off and that was causing the problem. I live in central America and I guess that the Tech used the incorrect program for the original alignment. Or he was seriously hung over the day that he first aligned the car.
Maybe you should ahve the alignement checked by another shop?
Insanity: doing the same thing over and over again and expecting different results.
2006 - Suzuki Gran Vitara (2.0 L fully equipped) Like this car so far except for trying to put on the seatbelt.
1988 - 190e - 2.3L - 172K miles (It now belongs to the exwife)
1999 - Chevy Blazer LS Fully Equiped - killed it June 2006
2001 - Honda Civic EX - 68K miles (sold June 2004)
1963 - 220S - Dual Carb 6 cyl. (sold)
1994 - Yamaha WaveRaider (fun to ride)